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Heating Calculator
Calculate your BTU for each room
How to use a heating calculator to get the right sized radiator
Selecting the correct heat output for your room is essential. A radiator that’s too small won’t warm the space effectively; one that’s too large may overheat the room and waste money on energy bills. Our simple heating calculator helps you find the ideal size quickly and accurately.
What you’ll need to measure
- Room dimensions: height, width, and length
- Window area: total surface area of all windows in the room
Tips for accurate measurements
- Measure to the nearest centimetre using a tape measure.
- Include all windows (bay, roof, and any glazed doors) in the window area.
- Older or poorly insulated rooms may require higher heat outputs.
Additional factors we consider
Accurate calculations aren’t just about room size. The calculator also asks:
- What’s above and below the room: loft, heated room, or unheated space
- Window type and glazing: single, double, or triple glazing
These simple questions, combined with your measurements, enable us to provide a precise recommended heat output so you can select a radiator with confidence.
Units we use
Radiator heat output is measured in British Thermal Units (BTUs) or Watts. Our calculator provides both values, and we display both on our product pages to make choosing your radiator or heated towel rail straightforward.
BTU (British Thermal Units)
Commonly used in the UK for radiator sizing.
Watts
Watts can describe both the input power to an electric element and the heat output delivered to the room. Power in Watts converted to heat becomes thermal output, typically expressed as Watts or BTU per hour.
Next steps
- Gather your measurements. Room height, width, length, and total window area.
- Answer the calculator’s questions. What’s above/below the room and your window glazing type.
- Use the BTU/Watt result. Filter and select a radiator or towel rail that matches the recommended output.
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